1. Field of the Invention
The present invention relates to a wireless communication system, a receiver, a demodulation method used in the system and receiver, and a program thereof, and particularly to a demodulation method in a receiver in a wireless communication system using plural transmission/reception antennae.
2. Description of the Related Art
In a wireless communication system in which a signal transmitted from a transmitter having plural transmission antennae is received by a receiver have plural reception antennae, studies have been made of a lot of techniques to reduce the scale of calculation in the receiver. With reference to FIG. 19, a description will now be made of a scheme according to a non-patent document 1 (Jiang Yue, Kyeong Jin Kim; Gibson, J. D.; Iltis, R. A., “Channel Estimation and Data Detection for MIMO-OFDM Systems,” In Proc. Of IEEE Intl. Conf. On Global Telecommunications Conference, Vol. 2, pp. 581-585, December 2003).
FIG. 19 is a block diagram showing the configuration of the scheme of the non-patent document 1. Referring to FIG. 19, this system is constituted by a transmitter 6 having four transmission antennae 21-1 to 21-4, and a receiver 12 having four reception antennae 11-1 to 11-4. Further, the receiver 12 has a QR-decomposition device 121, a QH-calculation device 122, a demodulation device 123, and a parallel-serial conversion device 54. The transmission antennae 21-1 to 21-4 of the transmitter 6 each transmit any of 16 values. The receiver 12 has four reception antennae 11-1 to 11-4 and respectively receive signals r1, r2, r3, and r4.
Suppose now that the channel coefficient of a signal path constituted by the transmission antennae 21-m (m is 1 to 4) and the reception antennae 11-n (n is 1 to 4) is hmn (h11 to h14, h21 to h24, . . . , h41 to h44). Transmission signals s1 to s4 and received signals r1 to r4 are related to each other, as expressed by the following formulas below with use of the channel coefficient hmn.r1=h11s1+h12s2+h13s3+h14s4+n1r2=h21s1+h22s2+h23s3+h24s4+n2r3=h31s1+h32s2+h33s3+h34s4+n3r4=h41s1+h42s2+h43s3+h44s4+n4  [Formula 1]
In these formulas, n1 to n4 respectively represents noises added to the received signals r1 to r4.
Suppose now that a vector having the received signals r1 to r4 as elements of itself is a received-signal vector r, a matrix having channel coefficients h11 to h44 as elements of itself is a channel matrix H, a vector having transmission signals s1 to s4 as elements of itself is a transmission-signal vector s, and a vector having as elements of itself noises n1 to n4 added to the received signals r1 to r4 is a noise vector n. Then, the above formula 1 can be expressed as follows, in a form using matrices.
                    r        =                              (                                                                                r                    ⁢                                                                                  ⁢                    1                                                                                                                    r                    ⁢                                                                                  ⁢                    2                                                                                                                    r                    ⁢                                                                                  ⁢                    3                                                                                                                    r                    ⁢                                                                                  ⁢                    4                                                                        )                    =                                                                      (                                                                                                              h                          ⁢                                                                                                          ⁢                          11                                                                                                                      h                          ⁢                                                                                                          ⁢                          12                                                                                                                      h                          ⁢                                                                                                          ⁢                          13                                                                                                                      h                          ⁢                                                                                                          ⁢                          14                                                                                                                                                              h                          ⁢                                                                                                          ⁢                          21                                                                                                                      h                          ⁢                                                                                                          ⁢                          22                                                                                                                      h                          ⁢                                                                                                          ⁢                          23                                                                                                                      h                          ⁢                                                                                                          ⁢                          24                                                                                                                                                              h                          ⁢                                                                                                          ⁢                          31                                                                                                                      h                          ⁢                                                                                                          ⁢                          32                                                                                                                      h                          ⁢                                                                                                          ⁢                          33                                                                                                                      h                          ⁢                                                                                                          ⁢                          34                                                                                                                                                              h                          ⁢                                                                                                          ⁢                          41                                                                                                                      h                          ⁢                                                                                                          ⁢                          42                                                                                                                      h                          ⁢                                                                                                          ⁢                          43                                                                                                                      h                          ⁢                                                                                                          ⁢                          44                                                                                                      )                                ⁢                                  (                                                                                                              s                          ⁢                                                                                                          ⁢                          1                                                                                                                                                              s                          ⁢                                                                                                          ⁢                          2                                                                                                                                                              s                          ⁢                                                                                                          ⁢                          3                                                                                                                                                              s                          ⁢                                                                                                          ⁢                          4                                                                                                      )                                            +                              (                                                                                                    n                        ⁢                                                                                                  ⁢                        1                                                                                                                                                n                        ⁢                                                                                                  ⁢                        2                                                                                                                                                n                        ⁢                                                                                                  ⁢                        3                                                                                                                                                n                        ⁢                                                                                                  ⁢                        4                                                                                            )                                      =                          Hs              +              n                                                          [                  Formula          ⁢                                          ⁢          2                ]            
The QR-decomposition device 121 estimates a channel matrix H by use of received signals r1 to r4, and carries out QR-decomposition. Suppose that the result thereof is H=QR. Q is a unitary matrix (hereinafter called a Q-matrix) and R is an upper triangular matrix (hereinafter called a R-matrix) having a real number of a diagonal element.
The QH-calculation device 122 multiplies the received-signal vector r by a complex conjugate transposition of the Q-matrix, thereby to output nulling signals z1 to z4 in which the received signals r1 to r4 are subjected to coordinate transformation. Note that the Q-matrix satisfies QHQ=I (I is a unit matrix). A nulling signal vector z which takes the nulling signals z1 to z4 as elements can be expressed as z=QHr=Rs+QHn. At this time, the R-matrix is an upper triangular matrix, and hence, the following is given.
                              (                                                                      z                  ⁢                                                                          ⁢                  1                                                                                                      z                  ⁢                                                                          ⁢                  2                                                                                                      z                  ⁢                                                                          ⁢                  3                                                                                                      z                  ⁢                                                                          ⁢                  4                                                              )                =                                            (                                                                                          r                      ⁢                                                                                          ⁢                      11                                                                                                  r                      ⁢                                                                                          ⁢                      12                                                                                                  r                      ⁢                                                                                          ⁢                      13                                                                                                  r                      ⁢                                                                                          ⁢                      14                                                                                                            0                                                                              r                      ⁢                                                                                          ⁢                      22                                                                                                  r                      ⁢                                                                                          ⁢                      23                                                                                                  r                      ⁢                                                                                          ⁢                      24                                                                                                            0                                                        0                                                                              r                      ⁢                                                                                          ⁢                      33                                                                                                  r                      ⁢                                                                                          ⁢                      34                                                                                                            0                                                        0                                                        0                                                                              r                      ⁢                                                                                          ⁢                      44                                                                                  )                        ⁢                          (                                                                                          s                      ⁢                                                                                          ⁢                      1                                                                                                                                  s                      ⁢                                                                                          ⁢                      2                                                                                                                                  s                      ⁢                                                                                          ⁢                      3                                                                                                                                  s                      ⁢                                                                                          ⁢                      4                                                                                  )                                +                      (                                                                                n                    ⁢                                                                                  ⁢                    1                                                                                                                    n                    ⁢                                                                                  ⁢                    2                                                                                                                    n                    ⁢                                                                                  ⁢                    3                                                                                                                    n                    ⁢                                                                                  ⁢                    4                                                                        )                                              [                  Formula          ⁢                                          ⁢          3                ]            
All of the elements (r1 to r4) constituting the received-signal vector r depend on the transmission signals s1 to s4. In contrast, of the elements (z1 to z4) constituting the nulling signal vector z, z4 depends only on the transmission signal s4, as well as z3 on the transmission signals s3 and s4, z2 on the transmission signals s2, s3, and s4, and z1 on the transmission signals s1 to s4.
Therefore, at the time of performing demodulation with use of a replica signal of a received signal, the number of replicas can be reduced more essentially in the case of using a nulling signal obtained by performing coordinate transformation on the received signal than in the case of directly using a received signal.
Generally in a maximum likelihood detection, all signal candidates that may have been used at the time of transmission are used to calculate replicas of received signals. The calculated replicas and actually received signals are compared with each other. Demodulation is carried out supposing that among replicas, the signal that gives the closest received signals was transmitted.
In contrast, in the scheme according to the non-patent document 1, replicas are generated in the order of the nulling signals z4 to z3 to z2 to z1. At this time, the number of signal candidates for which replicas of nulling signals are generated is limited to M, in each stage, thereby to reduce greatly the calculation amount (this is called a “M algorithm”).
The demodulation device 123 is inputted with a R-matrix and a nulling signal vector z and demodulates a signal by a maximum likelihood detection to which the M algorithm is applied. In this case, the number of errors that are selected by the M-algorithm and the number of signal candidates that give the errors are both 16.
At first, attention is paid to z4=r44s4+n4 in the above formula 3. Since n4 is a noise signal, it is very difficult for the receiver 12 to estimate n4. Hence, signal candidates of r44 and the transmission signal s4 are used to calculate a replica signal with respect to the nulling signal z4. The transmission signal s4 is a signal consisting of any of 16 values. If the values which the transmission signal s4 can take are c1 to c16, the replica signals of the nulling signals z4 can be calculated as r4-1=r44c1, r4-2=r44c2, . . . , r4-16=r44c16. Next, errors between calculated 16 replica signals r4-1 to r4-16 and the nulling signal z4 are obtained. Since the number of all signal candidates is 16, a selection processing based on the M-algorithm is not required.
Next, in the above formula 3, attention is paid to z3=r33s3+r34s4+n3. The transmission signal s3 is also constituted by any of 16 values. With use of r33, r34, and signal candidates of transmission signals s3 and s4, the replica signal of the nulling signal z3 can be calculated as z3-1=r33c1+r34c1, z3-2=r33c1+r34c2, . . . , z3-256=r33c16+r34c16. Errors between obtained replica signals z3-1 to z3-256 and the nulling signal z3 are obtained.
Since there are 256 sets of signal candidates for the transmission signals s3 and s4 at present, a selection processing of selecting 16 sets by the M-algorithm is required. This processing can be achieved by obtaining and selecting 16 sets of transmission signal candidates which have small errors with respect to nulling signals z3 and z4, with use of 16 replicas calculated for the nulling signal z4 and 256 replicas calculated for the nulling signal z3.
Next, replicas of the nulling signal z2 are calculated. At this time, the number of signal candidates for the transmission signals s3 and s4 is 16 sets. There are also 16 signal candidates for s2. Therefore, 256 replica signals of the nulling signal z2 can be calculated. In this case, a processing of selecting 16 replica signals by the M-algorithm is required, too. 256 replicas are calculated for the nulling signal z1. Together with 16 replicas of the nulling signals z2, z3, and z4, the most probable sets of signal candidates for the transmission signals s1, s2, s3, and s4 are finally selected and outputted as demodulated signals s1, s2, s3, and s4.
The parallel-serial conversion device 54 performs parallel-serial conversion on these demodulated signals s1, s2, s3, and s4, and outputs the conversion results as demodulated data s1, s2, s3, and s4 in a serial format.
In the case of using an error correction code or the like, the receiver 12 which calculates the bit likelihood calculates the likelihood from finally obtained signal candidates and errors thereof. The likelihood can be calculated as follows.
Usually, any of 0000 to 1111 is assigned to each of signals candidates c1 to c6 for each transmission signal. The bit likelihood means probability at which the bit at a particular position is 0 and probability at which the same bit is 1 where attention is paid to a particular bit position. This can be approximated by an error finally calculated according to the present scheme. For example, to obtain probability at which the bit at a bit position is 0, those signal candidates that have 0 at the bit position are searched for from signal candidates finally obtained. Among final errors calculated from the signal candidates, the smallest one is selected. This operation is performed repeatedly with respect to 0 and 1 at all bit positions, and the bit likelihood is obtained for every bit.
The related art described above requires that replica calculation should be executed 16+256+256+256=784 times in order to calculate demodulated signals or bit likelihood. With respect to z3 and z2, a selection processing of selecting 16 from 256 is required. With respect to z1, a selection processing of selecting 1 from 256 is required.
However, the related art as described above has problems as follows.
The first problem is that a lot of selection processings are necessary and cause a processing delay. This is because selection of signal candidates and errors is carried out by the M-algorithm.
The second problem is that bit likelihood cannot be calculated in some cases. This is because signal candidates finally obtained may cause a case that all the bits at a particular bit position are 0 or 1.
The third problem is that regardless of the states of channels, a constant processing is carried out. Therefore, setting of the numbers of remaining candidates is not always efficient. This is because the receiver does not carry out proper control corresponding to quality.